High-temperature X-ray diffraction study of uranium-neptunium mixed oxides

摘要

Incorporating minor actinides (MAs = Am, Np, Cm) in UO_2 fertile blankets is a viable option to recycle them. Despite this applied interest, phase equilibria between uranium and MAs still need to be thoroughly investigated, especially at elevated temperatures. In particular, few reports on the U-Np-O system are available. In the present work, we provide for the first time in situ high-temperature X-ray diffraction results obtained during the oxidation of (U_(1-y)Np_y)O_2 uranium-neptunium mixed oxides up to 1373 K and discuss subsequent phase transformations. We show that (i) neptunium stabilizes the UO_2-type fluorite structure at high temperature and that (ii) the U_3O_8-type orthorhombic structure is observed in a wide range of compositions. We clearly demonstrate the incorporation of neptunium in this phase, which was a controversial question in previous studies up to now. We believe it is the particular stability of the tetravalent state of neptunium that is responsible for the observed phase relationships.